The zebrafish is an excellent vertebrate model organism for both genetic and experimental embryological studies. We plan to use the zebrafish to study genetic control of important developmental pathways, such as hematopoiesis or blood formation. The study will include chemical mutagenesis of the DNA in sperm cells followed by genetic breeding and identification of fish with alterations in blood formation. These fish will be used to identify genes involved in the development of the blood. These studies may elucidate genes and their encoded proteins essential for hematopoiesis. Further, an understanding of the development of the blood components may help us to study the mechanism of leukemia formation. We identified a family of genes encoding the CCAAT enhancer binding proteins, or CEBPs, including a novel one in zebrafish that is expressed only in myeloid cells. This novel cebp gene may be important for the control of white cell development. Using gene mapping we identified a mutation in the gata1 gene in a